Vapor anode, multi-tube AMTEC converters, currently under development for possible use in future space power systems, utilize sodium working fluid. An avenue to improve the conversion efficiency, not seriously considered to date, is using potassium working fluid. This paper compares the performance and operation limits of Mo-41%Re, sodium and potassium AMTEC cells of an identical design. Unlike the sodium cell, the operation of the potassium cell is restrained by incipient dryout in the evaporator wick. Such incipient dryout could be delayed, but not fully avoided, by moving the evaporator wick surface further away from the cell's hot plate. The potassium cell with a remote evaporator delivers a peak electrical power (Pe) of 8.5 We at a load voltage V_L = 2.7 V and efficiency of 17.4%, when the hot side temperature, T_(hot) = 1174 K. The cell peak efficiency of 17.7% occurs at V_L = 3.0 V, but lower Pe (7.5 We) and T_(hot) (1136 K). When operated at a 50 K lower hot side temperature, the potassium cell delivers the same electrical power and voltage output as the sodium cell. For the same load voltage an hot side temperature, however, the potassium cell provides 1.2 We more power at a 2 percentage points higher efficiency than the sodium cell. Conversely, the optimized condenser temperature of the potassium cell (560 K) is 90 K lower than that of the sodium cell (650 K), which may increase the surface area of the heat rejection radiator by as much as 85%. Therefore, the choice of a potassium cell should be based on weighing the advantage of attaining higher performance and/or operating at lower T_(hot) versus increasing the surface area of the heat rejection radiator.
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